1. Field of the Invention
This invention relates generally to oxidation ditch wastewater treatment systems and, more particularly, to oxidation ditch wastewater treatment systems which address denitrification of wastewater.
2. Prior Art
Due to the desire to more completely treat wastewater before discharging it back into the ecosystem, an increasing emphasis has been placed on the removal of primary aquatic nutrients, phosphorus and nitrogen, during the wastewater treatment process. Because of increased concerns over nitrogen compounds released in wastewater treatment plant effluent, governmental nitrification requirements are being implemented more broadly than phosphorus removal.
In an oxidation ditch activated sludge wastewater treatment; system, wastewater is circulated around a continuous oxidation ditch, with air being added at some point in the ditch in order to encourage microorganisms to aerobically decompose the waste in the wastewater. The circulating aerated wastewater is known as mixed liquor. Typically, aeration is provided by devices such as vertical turbines, brush rotors, or air diffusors, which are positioned at a convenient point in the ditch. During aerobic decomposition the microorganisms utilize oxygen in the decomposition of the waste. Very little biological activity takes place with regard to nitrogen compounds during aerobic decomposition. However, as the mixed liquor moves away from the aeration device and oxygen is depleted by the microorganisms, an anoxic zone develops in a small portion of the mixed liquor in the ditch just prior to the point of aeration. If the influent to the system enters at the upstream limit of the anoxic zone, some of the wastewater carbon source is used for denitrification in the anoxic zone, with the microorganisms feeding on the nitrogen compounds in lieu of the absent oxygen compounds.
The limit of the anoxic zone is a function of the dissolved oxygen in the ditch, which varies according to the characteristics of the influent and the amount of aeration provided. Because of this variance, as well as the fact that the activated sludge process must typically strive to achieve a high dissolved oxygen content, the single anoxic zone is unable to achieve adequate nitrogen removal. Further, if intrachannel clarification is employed in the oxidation ditch, the anoxic zone can be in the area of the ditch used for intrachannel clarification. Denitrification can then occur within the intrachannel clarifier, with the released nitrogen gas causing sludge to float within the clarifier. Thus, external nitrogen removal systems are used in conjunction with the oxidation ditch. These external systems, such as batch reactor systems, require equipment, land and tank capacity external to the ditch as well as additional energy to establish flow between the oxidation ditch and the nitrogen removal system.